One pupil can be larger than the other because of an underlying issue, such as syphilis, stroke, brain aneurysm, multiple sclerosis and optical nerve tumor. The use of illicit drugs can also trigger this change, as stated by WebMD.
The most important thing to do is visit a doctor to know the underlying cause of the unequal size of pupils. In some people, this change can be accompanied by pain, vision loss, double vision, drooping eyelid and headache. Physical examination is necessary should a person have any of these symptoms. Treatment for the unequal pupil size is not necessary, but the underlying cause may need treatment.